Thermophotovoltaics (TPV) are p-n junction semiconductor devices, which convert thermal energy to electricity. Such devices are being envisioned as efficient converters of chemical to electrical energy, which are scalable to very small sizes. Compared to conventional photovoltaic system, TPV needs an emitter, which is heated by combustion of hydrogen sources and flammable gases. Radiated energy from a heated emitter operating at a lower temperature of approximately 2000° C. falls into the near infra-red (NIR) spectrum. In contrast to Photovoltaics (PV) systems (T˜6000° C.) that operate at the visible band, TPV cells require semiconductors with much lower band-gap. The antimonide-based cells such as Indium Gallium Arsenide Antimonide (InGaAsSb) or Gallium Arsenide (GaAs) are shown to have low band-gap values (Eg=0.35˜0.7 eV) suitable for TPV Cells. Current high-efficiency TPV cells must utilize a thick layer of a low bandgap semiconductor, as such materials at near-infrared have very low absorption coefficient. Also a photonic crystal-based emitter and filter structure mounted on a low bandgap semiconductor have been introduced to enhance the overall efficiency of TPV systems. A mm-size TPV system, where the photonic crystal emitter, InGaAsSb cells, and a maximum power point tracker are all integrated into a single device, has recently been fabricated and shown to present a power conversion efficiency of ˜2.2% and output power of 220 mW.
In this disclosure, the use of a low bandgap semiconductor materials, such as InGaAsSb with a bandgap of Eg=0.52 eV, in conjunction with a resonant nanoantenna is proposed in order to improve the performance of TPV cells. Instead of a bulk semiconductor that is commonly used for infrared energy conversion, a resonant nanoantenna loaded with a nano-meter size low bandgap semiconductor load has the potential to reduce a TPV system's volume and improve its efficiency due to the field enhancement at the antenna terminals. However, such low bandgap material at infrared frequencies possesses low conductivity and high permittivity with significant frequency dispersion. As a result, such a load cannot easily be matched to dipole antennas similar to those already considered in the literature. For a given low bandgap TPV semiconductor material and frequency band, a design procedure for a high impedance bowtie nanoantenna is presented. The combination of a frequency dependent load and the nanoantenna structure is simultaneously optimized to achieve the best thermal conversion to electricity. Here, a bowtie antenna topology is considered for achieving a higher intrinsic bandwidth and for the fact that such topology allows for simple connection of such TPV cells in series or parallel configurations. Thin nano-wires of resonant length can connect the ends of adjacent bowties for collecting DC current without affecting the infrared performance of individual nanoantennas. The antenna is operated at its anti-resonance mode (also referred to as parallel mode), instead of the series resonance mode normally used in RF antennas, to allow for load impedance matching at high levels. An extra transmission line stub placed in parallel with the semiconductor load can be utilized to compensate for the high capacitance of the load. It is emphasized here that the antenna impedance at the parallel resonance is purely resistive. Considering the InGaAsSb load and the parallel stub attached to it, then the maximum power transfer condition is met if the input admittance of the antenna is equal to that of the antenna load. The nanoantenna's dimensions including its length, angle, the size of the embedded load, and the parallel strip stub parameters are designed for the maximum power transfer.
This section provides background information related to the present disclosure which is not necessarily prior art.